Conventionally, a linear modulation technique is used in the packet-based WCDMA (Wideband Code Division Multiple Access) of the 3G UMTS (Universal Mobile Telecommunication System). In linear modulation, the amplitude of the modulation envelope is directly proportional to the amplitude of the modulated signal at all frequencies, and linear modulation techniques, such as e.g. QPSK (Quadrature Phase-Shift Keying) and 16 QAM (Quadrature Amplitude Modulation) are spectrum efficient and enables a high data rate.
However, a linear modulation requires more costly power amplifiers, since it results in a noise-like signal with a Rayleigh distributed amplitude, having a peak-to-average amplitude ratio, PAR, of about 10 dB, and the power amplifier of the RBS (Radio Base Station) has to be dimensioned to deliver the peak power. Various processing methods are commonly used to limit the PAR, and a typical PAR value after processing is about 7 dB. The radio coverage and the required number of sites is determined by the average transmitted power, and, consequently, a linearly modulated power amplifier has to be dimensioned to handle 7 dB more power than the average transmitted power. Thus, it is expensive to achieve a good coverage, which results in a limited 3G-system coverage, determined by the location of the indoor users.
The time dispersion, or delay spread, of modulation symbols may degrade a received signal, e.g. by intersymbol interference, ISI, where the energy from one symbol spills over into another symbol, and increase the BER (Bit Error Rate). The OFDM (Orthogonal Frequency Division Multiplexing) is a digital linear multi-carrier modulation scheme used in the 3G-LTE (Long Term Evolution), comprising a large number of closely-spaced orthogonal sub-carriers, each sub-carrier modulated with a conventional linear modulation scheme, e.g. QPSK, 16 QAM or 64 QAM, depending on the radio conditions. The OFDM has very long symbols, i.e. ten times longer than the maximum time dispersion, and, consequently, the degradation caused by time dispersion is negligible, requiring no delay equalizer.
On the contrary, a non-linear constant envelope modulation technique, such as e.g. the GMSK (Gaussian Minimum Shift Keying) commonly used in the GSM (the Global System of Mobile communication), has very short symbols, shorter than one tenth of the length of the maximum time dispersion, and a delay equaliser is normally used in order to reduce the time dispersion. The GMSK results in a comparatively low bit-rate, and an increased output power and coverage, with a PAR of 0 dB, but the existence of spectrum emission outside the operating frequency band will lead to poor spectrum efficiency. The GMSK is a type of frequency-shift keying with a continuous phase, and the signal to be modulated is filtered in a Gaussian low-pass filter before being fed to a frequency modulator, thereby reducing the adjacent channel interference.
In order to increase the bit-rate resulting from the GMSK, the conventional GSM/EDGE is adapted to switch to a linear 8 PSK modulation from the constant envelope GMSK modulation, depending on the monitored radio conditions. Since the linear 8 PSK suffers the drawbacks of a lower average output power and a reduced coverage, 8 PSK is only used when the radio conditions are good, otherwise the constant envelope GMSK is maintained.
The packet-based W-CDMA of the 3G uses linear modulation, and handles the time dispersion by a RAKE-receiver, since an equalizer is only applicable at a lower bit-rate. The linear OFDM-modulation of the 3G-LTE, for the future 4G, has a negligible degradation due to time dispersion. However, even though the linear OFDM-modulation of the LTE results in a high bit rate, it has the drawbacks of a limited coverage and a non-efficient use of the power-amplifiers, and the above-described solution of the GSM/EDGE is not applicable, due to the much larger bandwidth of the LTE. Thus, it still presents a problem to achieve an improved coverage and a more efficient use of the power-amplifier in a packet radio communication channel, in particular in a multi-user environment, without increasing the time dispersion degradation.